1. Technical Field
The present invention relates to a closed type battery such as a lithium ion battery provided with a covering film having a safety valve and an assembled battery with such lithium ion batteries connected in series and/or parallel, and to a structure adapted to ensure that the safety valve is put in operation upon generation of gases by abnormal charging or the like.
2. Related Art
Lithium ion batteries, because of being more lightweight and higher in energy density than other secondary batteries such as nickel-cadmium or nickel-hydrogen ones, are now increasingly used just only as drive power sources for portable terminals such as cellular phones, notebook PCs and video cameras, but also as power sources for electric cars or vehicles.
A lithium ion battery is built up of a negative electrode formed of carbon or other material capable of doping and dedoping lithium ions, a positive electrode formed of a lithium-containing transition metal oxide such as lithium cobaltate, lithium nickelate and lithium manganate, and an electrolysis solution composed of an organic solvent with a lithium salt such as LiPF6 dissolved in it. When that organic electrolysis solution is placed in a voltage area much greater than 4.2 V, it often gives out combustible gases, etc. by way of oxidative breakdown, causing the battery to generate heat or break due to an increase in the battery's internal pressure.
Applications with a multiplicity of lithium ion batteries connected in series and parallel, for instance, power sources for electric bicycles, electric cars, etc. and uninterruptible power sources have grown large. When a covering film with a synthetic resin film and an aluminum foil or the like laminated together is used as a covering member for a plurality of batteries, it is possible to obtain an assembled battery greater in capacity density and weight density than that obtained using a metal can as the covering member.
With the generation of gases, however, a lithium ion battery fabricated using a flexible covering film swells or inflates a lot more than does one fabricated using a metal can as the covering member. For the lithium ion battery using a covering film as the covering member, there have thus been numerous proposals made of a safety valve adapted to expel the generated gases from within it.
For a battery with a covering film used as the covering member, two covering films, each having a three-layer structure of nylon film/aluminum foil/polypropylene film, are used. The battery is sealed up around by the heat fusion of the polypropylene layers. When the battery is in an abnormal state for the reasons of overcharging or the like, gases are generated by the decomposition of an electrolysis solution or otherwise battery elements generate heat. In turn, it will cause the heat fusion seal to break open or even burst up.
In this conjunction, JP-A-1999-086823 and 1999-097070 have come up with providing a part of the seal with a site having pressure resistant performance lower than the rest or providing the heat fusion seal with a site having a lower peel strength, so that such sites can cleave selectively.
JP-A-1999-312505 has proposed a low-profile battery having a safety valve wherein the innermost resin layer of a covering film is made partially thin enough to cleave upon an increase in the internal pressure, and JP-A-2000-100399 has come up with making the heat fusion temperature for a seal area functioning as a safety valve lower than that for a seal area having no safety valve.
Further, JP-A-2005-116474 has proposed a battery wherein a laminated film is used as a covering film, and a safety valve having a valve body comprising a low-melting resin is located at a part of a heat fusion seal with a heat conduction member capable of conducting heat generated from battery elements directly to the safety valve.
A lithium ion battery with a flexible covering film applied on it is shown in FIG. 11. A stack member that becomes a battery element with a positive 2 and a negative electrode terminal 3 drawn out of it is covered at its top side with an embossed covering film and at its bottom side with a flat covering film. A periphery 6 is heat fused in order for each side by a hot plate or a frame-form hot plate.
A safety valve 7 is provided at the fusion side from which the positive 2 and the negative electrode terminal 3 are not drawn. When a plurality of such batteries as fabricated as mentioned above are connected together and used, such a safety valve fails to perform its own function, because portions of the seal other than the safety valve break open under the influences of heat and gases generated from nearby batteries.
Referring especially to a lithium ion battery using a covering film with the positive 2 and the negative electrode terminal 3 for drawing currents mounted at the same seal side, a sealant material is located at the positive electrode terminal 2 made of aluminum and the negative electrode terminal 3 made of nickel or copper to prevent leakages and increase fusion strength. However, when there is heat generated from the battery, the heat fusion seal of the periphery 6 may often break open between both the terminals, because the seal portion contacting the metal positive or negative electrode terminal becomes higher in temperature than other seal portions.